A diesel particulate filter (DPF) system cleans up an exhaust gas emitted by an engine. The DPF system typically includes a pressure sensor that detects a pressure difference of an emitted exhaust gas between before and after passing through a DPF. The DPF works as a filter, and the pressure sensor is used for checking, for example, an occurrence of a DPF clogging. A pressure sensor like the above-described sensor may be used in a pressure medium such as the exhaust gas having high corrosive property. Thus, a sensor structure is important to protect a sensor chip against a corrosive pressure medium. The sensor chip detects the pressure.
A pressure sensor structure is taught in U.S. Pat. No. 6,651,508. The pressure sensor includes a gel for protecting a sensor chip of the pressure sensor. More specifically, the pressure sensor comprises a resin casing. The resin casing includes a pressure introduction port having a first pressure introduction passage and a second pressure introduction passage. The first pressure introduction passage and the second pressure introduction passage introduce a pressure medium therein. The pressure sensor further comprises a semiconductor diaphragm-type sensor chip. The chip is a plate shaped sensing member having a first surface and a second surface. The first surface of the sensing member receives the pressure of the pressure medium introduced in the first pressure introduction passage. The second surface of the sensing member receives the pressure of the pressure medium introduced in the second pressure introduction passage. The difference of the pressures applied to the first surface and the second surface can be measured.
A pressure sensor like the above-described sensor includes a glass substrate having a first through-hole. The resin casing has a second though-hole. The resin casing is bonded to the glass substrate. The resin casing is connected to the first though-hole of the glass substrate. Further, the second surface of the sensor chip is bonded to the glass substrate. When the sensor chip has a dimensions, for example, 3 mm square, the size of each through-hole is given as, for example, 1.25 mm.
A first protecting member is disposed on the first surface of the sensor chip. A second protecting member is disposed in the first and second though-holes. Material of the first and second protecting members is gel that can protect the sensor chip against the pressure medium having a corrosive property.
Since the second surface of the sensor chip is covered by the gel, the resin casing needs to have the second though-hole for disposing the gel therein. In view of strength of the resin casing, it is considered that thickness of the resin casing is, for example, at least 1 mm.
In view of the above-described case, the present inventor has clarified that there arise following problems.
FIG. 3 illustrates a pressure sensor according to a related art. A sensor chip 92 is coupled with a resin casing 93 via a base 91, and a second surface receives a pressure of a pressure medium. The resin casing 93 includes a third through-hole 94 having a hole size φ, e.g. a hole diameter, and a hole length t. The hole length t is, for example, a distance between an end of the third though-hole 94 and the second surface of the sensor chip 92, as shown in FIG. 3. FIG. 4 shows a relation between a bending ratio of a temperature characteristic and φ/t. The bending ratio of a temperature characteristic is also referred to herein as BRC.
The BRC expresses a sensor characteristics difference between following two cases in a low temperature region; one case is that a gel member 95 covers the second surface of the sensor chip 92 with a penetration degree of 120, and the other case is that the gel member 95 does not cover the second surface of the sensor chip 92. The penetration degree associates with hardness of the gel member 95. For example, when the penetration degree is 100, the gel member 95 is soft. When the penetration degree is 40, the gel member 95 is very hard for example.
As shown in FIG. 4, as φ/t is larger or as a diameter of the third though-hole 94 is larger, the BRC is smaller, and the sensor has a better characteristic. However, as, as φ/t is smaller or as the diameter of the third though-hole 94 is smaller, the BRC is larger, and the sensor has a worse characteristic. As described above, when the resin casing 93 includes the third though-hole 94, the resin casing 93 needs to have a thickness enough to ensure the strength of the resin casing 93. Since the hole size of the though-hole is inevitably smaller than a size of the chip sensor 92, it is likely that a pressure sensor is designed such that third though-hole 94 has a smaller diameter than the length of the third though-hole 94. Therefore, there arises a problem that the BRC is large and a characteristic error of the sensor chip 92 is large, as shown in FIG. 4.
It is considered that the third though-hole 94 is sealed with using the gel member 95 made of soft material. FIG. 5 is a graph showing a relation between the gel hardness and a deviation before and after gel inserting. The deviation before and after gel inserting indicates a degree of a sensor characteristic deviation. Specifically, as the deviation before and after gel insert is closer to 0%, the sensor has fewer characteristic deviation. As the deviation before and after gel insert has a larger value, the sensor has larger characteristic deviation, which corresponds to the sensor having a worse characteristic.
As shown in FIG. 5, as the gel member 95 is harder, the sensor has larger characteristic deviation corresponding to the sensor having a worse characteristic. However, as the gel member 95 is softer, the sensor has smaller characteristic deviation, which corresponds to the sensor characteristics being not influenced.
The resin casing 93 has a rough surface caused by a filler such as that made of glass included in the resin casing 93 and caused by a formation condition of the resin casing 93. Thus, there arises a condition that an air bubble can be attached to a surface of the resin casing 93. When the gel member 95 made of the soft material is applied to the resin casing 93 in the condition as described-above, the air bubble penetrates an inner potion of the gel member 95. There arises a problem that the air bubble prevents from transmitting a pressure, and thereby a worse sensor characteristic is caused.
Further, there arise problems that a thermal expansion coefficient difference between the gel member 95 and the resin casing 93 causes the generation of stress, and that the gel member may peel off from the resin casing 93. In the above-described case, the emitted exhaust gas having the corrosive property may penetrate a boundary or a space between the gel member 95 and the resin casing 93. The emitted exhaust gas may decompose the gel member 95 and the resin casing 93.